net/mlx5e: Add mdev to flow_steering struct

Make flow_steering struct contain mlx5_core_dev such that
it becomes self contained and easier to decouple later on this series.
Let its values be initialized in mlx5e_fs_init().

Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>